Rapastinel (GLYX-13) is an N-methyl-D-aspartate receptor (NMDAR) modulator that has characteristics of a glycine site partial agonist. Rapastinel is a robust cognitive enhancer and facilitates hippocampal long-term potentiation (LTP) of synaptic transmission in slices. In human clinical trials, rapastinel has been shown to produce marked antidepressant properties that last for at least one week following a single dose. The long-lasting antidepressant effect of a single dose of rapastinel (3 mg/kg IV) was assessed in rats using the Porsolt, open field and ultrasonic vocalization assays. Cognitive enhancement was examined using the Morris water maze, positive emotional learning, and contextual fear extinction tests. LTP was assessed in hippocampal slices. Dendritic spine morphology was measured in the dentate gyrus and the medial prefrontal cortex. Significant antidepressant-like or cognitive enhancing effects were observed that lasted for at least one week in each model. Rapastinel facilitated LTP 1 day – 2 weeks but not 4 weeks post-dosing. Biweekly dosing with rapastinel sustained this effect for at least 8 weeks. A single dose of rapastinel increased the proportion of whole-cell NMDAR current contributed by NR2B-containing NMDARs in the hippocampus 1 week post-dosing, that returned to baseline by 4 weeks postdosing. The NMDAR antagonist CPP blocked the antidepressant-like effect of rapastinel 1 week post dosing. A single injection of rapastinel also increased mature spine density in both brain regions 24 hrs post-dosing. These data demonstrate that rapastinel produces its long-lasting antidepressant effects via triggering NMDAR-dependent processes that lead to increased sensitivity to LTP that persist for up to two weeks. The data also suggest that these processes led to the alterations in dendritic spine morphologies associated with the maintenance of long-term changes in synaptic plasticity associated with learning and memory.

Structural studies have shown that chronic regimens of psychostimulants increase dendritic spine number in the rat striatum. The present study used Western blotting and radioimmunocytochemistry to examine psychostimulant-induced changes in the levels of spinophilin, a protein found abundantly in dendritic spines. Spinophilin determinations were conducted in striatum as well as several other subcortical regions implicated in psychostimulant-induced neuroplasticity. Rats received an escalating (1-8 mg/kg) dosing regimen of d-amphetamine (twice daily, i.p.) for 5 weeks, were tested for locomotor sensitization, and were killed 28 days later. This amphetamine dosing regimen was found to induce a significant sensitization of locomotor activity in these animals. Using both Western blotting and radioimmunocytochemistry, spinophilin protein was found to be upregulated in the striatum of amphetamine-treated rats. In addition, radioimmunocytochemistry revealed that spinophilin was increased in the septum, hippocampus, amygdala and the cingulate cortex, and was unchanged in sensorimotor cortices. Because it binds to F-actin and protein phosphatase-1, spinophilin has been proposed as a protein linking synaptic transmission to changes in spine morphology. Radioimmunocytochemistry for spinophilin provides a novel approach to identification of brain regions whose neurons undergo dendritic change after chronic exposure to drugs of abuse.

Early life nutrition plays an important role in brain development. Emerging research in rodents, piglets and humans suggest that prebiotics, milk fat globule membrane and lactoferrin may each play unique roles in brain development and cognitive functions. However, knowledge of their combined impact is lacking. We show here that providing weanling rats with a diet containing milk fat globule membrane, lactoferrin and a polydextrose/galactooligosaccharide prebiotic blend led to a significant increase in total dendritic spine density in hippocampal dentate gyrus neurons. Region-specific alterations in dendritic spine density and morphology could provide a mechanistic basis underlying broader cognitive benefits, but further research is required to demonstrate functional consequences of these observations.

Numerous studies in this lab and others have reported psychostimulant-induced alterations in both synaptic protein expression and synaptic density in striatum and prefrontal cortex. Recently we have shown that chronic d-amphetamine (d-AMPH) administration in rats increased synaptic protein expression in striatum and limbic brain regions including hippocampus, amygdala, septum, and paraventricular nucleus of the thalamus (PVT). Potential synaptic changes in thalamic nuclei are interesting since the thalamus serves as a gateway to cerebral cortex and a nodal point for basal ganglia influences. Therefore we sought to examine drug-induced differences in synaptic protein expression throughout the diencephalon. Rats received an escalating (1-8 mg/kg) dosing regimen of d-AMPH for 5 weeks and were euthanized 28 days later. Radioimmunocytochemistry (RICC) revealed significant upregulation of both spinophilin and the vesicular glutamate transporter, VGLUT1, in PVT, mediodorsal (MD), and ventromedial (VM) thalamic nuclei as well as in lateral hypothalamus (LH) and habenula. Strong positive correlations were observed between VGLUT1 and spinophilin expression in PVT, medial habenula, MD, VM and LH of d-AMPH-treated rats. No significant d-AMPH effect was seen in sensorimotor cortices for either protein. Additionally, no significant differences in the general vesicular protein synaptophysin were observed for any brain region. These findings add to evidence suggesting that long-lasting stimulant-induced synaptic alterations are widespread but not ubiquitous. Moreover, they suggest that d-AMPH-induced synaptic changes may occur preferentially in excitatory synapses.